1. Field of the Invention
The invention is related in general to the area of inspection systems. In particular, the present invention is related to method and systems for detecting defects the dimensions of which may exceed the optical resolution limits of a given imaging system. The present invention may be advantageously used in fabrication processes for devices such as flat panel displays, lithography masks, and semiconductor wafers.
2. The Background of Related Art
Driven by the demand for performance enhancement and cost reduction, the design rule (DR) of integrated circuits (IC) shrinks every year as the technology of manufacturing IC progresses. In the current manufacturing process of IC volume production, pattern represent designs are imprinted on silicon wafers using optical lithography techniques. The minimum dimension of a pattern that can be imprinted may be well below the theoretical optical resolution limit. Various resolution enhancement technologies implemented in the lithography optics, mask designs, and resist engineering have overcome the optical resolution limit. However, the dimensions of various defects that may occur during the manufacturing process become smaller and smaller as the DR shrinks. The smallest defects, but critical to the performance of finished IC chips, that need to be detected and identified during the manufacturing process are only a fraction of the DR. The rapid progress in IC manufacturing technology has generated very demanding requirements on optical defect inspection.
The ability of optical inspection to detect the smallest defect is largely determined by a signal-to-noise ratio of the defect. The signal of a defect is to the first order determined by the optical resolution of an inspection tool, and to the second order by a number of parameters of the optics configuration, such as wavelength, the geometry of illumination and collection, polarization, and the modes of imaging. While the signal of a defect increases with the higher optical resolution, there are both theoretical and practical limits of the highest optical resolution that can be used in an optical inspection tool. As the limits of optical resolution are approached, other means of improving defect detection sensitivity have to be implemented and become critical in meeting the requirements of defect inspection in production line.
Two of the commonly used imaging modes in the conventional microscopy, bright field and dark field, have been applied to defect inspection in IC manufacturing process. Bright field imaging has the advantages of higher optical resolution since it can use the maximum numerical aperture (NA) of the imaging lens. However, the bright background of the optical image tends to reduce the dynamic range and results in higher background noise. On the other hand, dark field imaging has the advantages of reducing or eliminating the background, therefore highlighting defects. Although a portion of the NA space is used for the illumination in dark field, which results in a smaller usable NA for imaging, therefore lower optical resolution, an enhancement of the signal-to-noise of defect due to the reduction of background often makes up more than enough the loss of optical resolution. In addition, less amount of data to be acquired and processed at lower resolution often leads to a higher inspection speed and lower system cost. Nevertheless, one of the disadvantages of dark field imaging is that it generally requires high brightness light source for illumination, since the scattered light collected in dark field imaging is only a small fraction of the incident light.
Light sources used for defect inspection imaging include both broad band lamps and lasers. Lamps have lower brightness and are often inadequate in providing enough light intensity for dark field defect inspection, especially at high resolution where imaging pixel is small and number of photons per pixel is lower. Lasers have the highest brightness among various light sources, and their wide applications have helped the development of commercially available lasers that can generate intense light at various wavelengths, from infrared to deep ultra-violet (UV). Lasers are not only used for dark field inspections, but also for bright field inspection when lamps do not have enough brightness at a desired wavelength, especially in the deep UV range. These types of inspection technologies are disclosed in U.S. Pat. No. 6,943,876, U.S. Pat. No. 6,693,664, and U.S. Pat. No. 6,288,780.
Conventional dark field inspection uses an expanded beam to illuminate a field of view, the NA of the illumination beam is much smaller than the NA of the imaging lens. In most cases, it is close to zero when the laser beam is nearly collimated. There is a significant disadvantage of using nearly collimated laser beam for illumination, it tends to cause excessive noise in imaging a surface that is not perfectly smooth. In most of the intended applications of defect inspection, a substrate is coated with a thin film that is inherently rough on surface, or patterned with multiple layers of structures of the integrated circuits. When the NA of the illumination beam is very small, or the nearly collimated, the wave front of the illumination beam over the entire optical field of view is substantially coherent. As a result, the interference between neighboring scatters on the substrate amplifies any small variations of the pattern on substrate which increases the noise floor and degrades defect detection sensitivity.
An example of such noise is the speckle generated from a diffusive surface when illuminated by a laser. For defect inspection during IC manufacturing process, the excessive noise can be significant. Some of the materials used in IC manufacturing are inherently grainy, and the surface is rough when coated a thin film on wafer surface (e.g., poly silicon and metal). In addition, the process of generating an IC pattern on wafer has some small random variation while the dimension of the pattern printed on wafer may also have small variation. Defects are identified by comparing identical features printed on wafer, such as two identical dies or cells in memory device. The small variation is often the limiting noise in detecting defect. In the case of coherent imaging system, the small variation is amplified through the interference effect and reflected as excessive noise in the die to die or cell to cell comparison process.
Suppressing noise can improve defect detection sensitivity as effectively as enhancing defect signal, especially when the optical system operates near its theoretical limits. There is thus a great need for techniques to provide an effective way of reducing the excessive noise in imaging.